Input: axp20x-pek - avoid needless newline removal

This code is doing more work than it needs to.

Before handing off `val_str` to `kstrtouint()` we are eagerly removing
any trailing newline which requires copying `buf`, validating it's
length and checking/replacing any potential newlines.

kstrtouint() handles this implicitly:
kstrtouint ->
  kstrotoull -> (documentation)
|   /**
|    * kstrtoull - convert a string to an unsigned long long
|    * @s: The start of the string. The string must be null-terminated, and may also
|    *  include a single newline before its terminating null. The first character
|    ...

Let's remove the redundant functionality and let kstrtouint handle it.

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230925-strncpy-drivers-input-misc-axp20x-pek-c-v2-1-ff7abe8498d6@google.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>